1. Technical Field
The present invention is generally related to the synchronization of clocks that are separated.
2. Description of the Related Art
High-accuracy synchronization of clocks plays an important role in fundamental physics and in a wide range of applications such as communications, message encryption, navigation, geolocation and homeland security. A classical method of time synchronization of spatially separated clocks is Eddington slow clock transport. In this approach, two co-located clocks are initially synchronized, and then one of the clocks is slowly transported to another location to synchronize with a distant clock, i.e., a geographically separated clock. For most technological applications, this method is not practical because it requires transport of hardware, i.e., the clock, as well as conflicting requirements: on the one hand, clock transport must be slow to reduce the relativistic effect of time dilation, but on the other hand, the transport must be fast enough so that significant time differences do not accrue from unavoidable timing errors due to the limited frequency stability of the transported clock's mechanism or due to gravitational potential differences along the path of the transported clock.
Today, in practical applications, a satellite system, such as the Global Positioning System (GPS), is used for synchronizing two spatially separated clocks. GPS is a satellite system in which signals are sent from satellite-to-ground and from ground-to-satellite to synchronize the satellite clocks with a master clock on the Earth. The time-synchronization accuracy provided by a GPS receiver is on the order of 20 nanoseconds (ns). However, there are applications, such as coherent detection of high-frequency electromagnetic signals, where time synchronization is required to an accuracy that cannot be provided by GPS. Therefore, there exists a need for synchronizing spatially separated clocks to an accuracy better than the nanosecond range